When you leave San Antonio you are about 3-4 hours to Del Rio, from there you are 3-4 hours from Alpine, from there you are 3-4 hours from Van Horn.
You will find I90 a good road but it does have large areas of open road.
It's all about preparation; take the time to research your travel route using Google Maps, set up your fuel and overnight camping, check your RV tires, your tow vehicle oil, fuel mileage, radiator, tires. You shouldn't have any problems getting to Alpine, but after Alpine if you are going on west past Van Horn and on into New Mexico things don't get any less remote.
The upside is you will see some great scenery, meet some of the best people anywhere, and get to visit some historic towns.
